In the treatment of wood material to render it resistant to biological attack, it is common practice to treat it with a fluid-borne treating chemical. The treatment of wood material usually comprises soaking or impregnating the wood material with the fluid-borne treating chemical. The second of these techniques which is perhaps the most widely used one is practised in a number of variants, e.g. the full cell process, where the pores of the wood material are partially evacuated before impregnation in order to increase the fluid retained therein; or the empty cell process which is similar to the full cell process except that the preliminary step of evacuation is omitted.
One of the more widely used impregnating compositions is an acidic solution of chromium, copper and arsenic (CCA). Another aqueous composition commonly used in the impregnation of wood material, comprises an ammoniacal solution of water-insoluble treating chemicals such as salts comprising copper and arsenic. While these compositions are generally satisfactory, they both contain arsenic.
Prior to applicant's filing of the application, the inventor was investigating means to obtain a wood preservative system free of arsenic but substantially as effective as those containing arsenic. J. A. Butcher disclosed in the New Zealand Journal of Forestry Science, Vol. 9, p. 348, 1979, systems containing quaternary ammonium compounds and copper. He summarized his findings by pointing out two major problems, one of these being that the copper salts caused very serious corrosion problems. At that time, those working on quaternary ammonium compounds were working on systems which were either neutral or acid.
The reasons for using acidic or neutral systems were based upon the fact that penetration of quaternary ammonium compounds into wood in the presence of Na.sub.2 CO.sub.3 had been demonstrated to be hindered: In the New Zealand Journal of Forestry Science, Vol. 8, No. 3, p. 403 (1978), Butcher and Drysdale stated in an article entitled "Efficacty of Acidic and Alkaline Solutions of Alkylammonium Compounds as Wood Preservatives" that the question, according to them was one of pH, in that treatment with acidic solutions resulted in even distribution of alkylammonium compounds throughout the wood, such as is required for effective wood preservation, whereas treatment with alkaline solutions resulted in primarily adsorption in the surface layers. The failure of alkaline solutions of alkylammonium compounds to adequately protect the interion zones of wood is also discussed in Butcher, Preston, Hedley and Cross, New Zealand Forest Service Reprint, No. 1111, p. 6, 1977.
Furthermore, in Forest Products Journal, Vol 29, No. 11, p. 26 (1979), Hulme and Thomas specifically used Na.sub.2 CO.sub.3 to enhance surface soprtion of quaternary ammonium compounds used in anti-sapstain treatments where, in contrast to wood preservation application, such surface localization is desirable. This, again, points away from the use of alkaline systems for wood preservation with quaternary ammonium compounds.
Contrary to the technology of record and the direction indicated thereby, applicant has found that an ammoniacal formulation comprising copper and/or zinc and a quaternary ammonium compound is an effective wood preservative with none of the corrosion problems associated with acidic or neutral metal-containing systems of this type.
It is an object of this invention to provide a method for effectively treating wood in order to obtain a wood product having low mammalian toxicity.